1. Field of the Invention
The present invention relates to an active matrix substrate and a method of manufacturing the same.
2. Description of Related Art
An active matrix type display element is known as one of electro-optic display elements such as liquid crystal or organic EL. In the active matrix type display element, a switching element such as TFT is arranged on one principal surface of an insulating substrate, and independent voltage and electric field are applied to each pixel.
In the electro-optic display elements, it is important to realize the active matrix substrate in which a display area of each pixel is as large as possible, or aperture ratio is high, in order to realize clear display with high quality.
Now, a method of manufacturing a TFT array substrate which is an active matrix substrate will be described with reference to FIG. 10. FIG. 10 is a cross sectional view schematically showing the structure of a TFT array substrate 100. First, a conductive film is formed on an insulating substrate 1. Then, the conductive film is patterned by a photolithography process. Thus, a gate electrode 2, a gate signal line, and an auxiliary capacitance electrode 4 are formed on the insulating substrate 1. Next, a first insulating film 3 and a second insulating film 5 are successively formed. Next, a semiconductor active film 6 and an ohmic contact film 7 are successively formed. Then, the semiconductor active film 6 and the ohmic contact film 7 are patterned by the photolithography process.
Next, a metallic thin film is formed. Then, the metallic thin film is patterned by the photolithography process. Thus, a source electrode 9, a source signal line 110, and a drain electrode 10 are formed on the ohmic contact film 7. Then, the ohmic contact film 7 is etched by using the pattern of the drain electrode 10, the source signal line 110, and the source electrode 9 as a mask. Thus, a channel portion 8 is formed. Next, a third insulating film 11 is formed so as to cover them. Then, the third insulating film 11 is patterned by the photolithography process. Thus, a contact hole 12 is formed on the drain electrode 10. After that, the conductive film is formed. Then, the conductive film is patterned by the photolithography process. As such, a pixel electrode 13 that is connected to the drain electrode 10 by the contact hole 12 is formed. The TFT array substrate 100 is thus manufactured.
As stated above, when the active matrix substrate is formed by five photolithography processes, the gate electrode 2, the gate signal line, and the auxiliary capacitance electrode 4 are formed in the same process at the same time. The method of forming them at the same time is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 10-268353. In such a case, the gate electrode 2, the gate signal line, and the auxiliary capacitance electrode 4 are formed by the conductive film of the same material. The gate electrode 2 and the gate signal line are preferably made of low-resistance material so as not to delay the gate signal. Thus, the metallic thin film should be used as the gate electrode 2 and the gate signal line. Thus, the gate electrode 2, the gate signal line, and the auxiliary capacitance electrode 4 are formed of the metallic thin film. As the metallic thin film interrupts light, there is a limit to increase the aperture ratio. Further, in the method described above, the gate electrode 2, the gate signal line, and the auxiliary capacitance electrode 4 are formed in the same layer, which may cause short defect.